According to technological development and increased demand for mobile devices, demand for using secondary batteries as energy sources has rapidly increased. Among such secondary batteries, lithium secondary batteries having high energy density and operating potential, long cycle life, and low self-discharge rate are commercially available and widely used.
A lithium secondary battery has a structure in which an electrode assembly including a positive electrode, a negative electrode, and a porous separator disposed between the positive electrode and the negative electrode is impregnated in an electrolyte solution including a lithium salt, wherein the positive electrode and the negative electrode are each prepared by applying an active material on an electrode current collector. During a charging process, lithium ions of a positive active material are dissolved and inserted into an active material layer of the negative electrode. During a discharge process, lithium ions in the active material layer are dissolved and inserted into the positive active material. The electrolyte solution serves as a medium that transfers lithium ions between the negative electrode and the positive electrode.
The electrolyte solution generally includes an organic solvent and an electrolyte salt. For example, the electrolyte solution may be prepared by adding a lithium salt, such as LiPF6, LiBF4, or LiClO4, in a solvent mixture including high-dielectric cyclic carbonate, such as propylene carbonate or ethylene carbonate; and low-viscosity chain carbonate, such as diethyl carbonate, ethyl methyl carbonate, or dimethyl carbonate.
However, when a copper foil is used as a negative electrode current collector, the problem of copper ions (Cu2+) being dissolved from a copper current collector into an electrolyte solution occurred in cases of overdischarging the lithium secondary battery or exposing the lithium secondary battery to a high-temperature environment, and this became a cause for deteriorating stability of the negative electrode.
Particularly, when the lithium secondary battery is overdischarged such that a voltage of the battery becomes 0 V, a voltage of a side of the negative electrode having a large irreversible capacity first increases, and when the voltage of a side of the negative electrode reaches a predetermined voltage region of about 3.6 V or higher, where the copper foil oxidizes, copper ions are dissolved from the copper foil into the electrolyte solution.
The dissolved copper ions precipitate back to a metal on a surface of the negative electrode during a charging process and thus deteriorate stability of the negative electrode. For example, precipitated copper may generate a fine short circuit on the surface of the negative electrode or may disturb intercalation of lithium ions and thus may decrease a charging and discharging capacity.
As a method to resolve these problems, Patent Document 1 (KR 2006-0063749) discloses an electrolyte solution for a lithium secondary battery, the electrolyte solution including a multi-component metal oxide salt represented by Formula AxMyOz (where, A is at least one element selected from the group consisting of alkali metals and alkaline earth metals, M is at least one element selected from the group consisting of nonmetals, semimetals, and transition metals, 1≤x≤6, 1≤y≤7, and 2≤z≤24), e.g., a metal oxide salt such as Li2MoO4, Li2WO4, as an additive. However, when this method is used, although an oxidation initiating voltage increases a little, the problem of dissolution of copper ions occurring when the lithium secondary battery is left at a high temperature could not have been resolved.
In addition to the method of adding a new material to an electrolyte solution, a method of chaining elements such as an electrode plate or a separator may be used. For example, Patent Document 2 (JP 2005-063764) discloses a method of preventing dissolution of copper during an overdischarge process by providing a copper foil for a lithium ion secondary battery, wherein a chrome-based thin layer is formed on a surface of the copper foil. However, a process of the method is complicated, and the method is not efficient in terms of cost, compared to those of the method of adding an additive to an electrolyte.